In the drilling of wells, drilling mud is circulated through the drill string to contain the well, lubricate the bit, and remove cuttings from the bore hole. The drilling mud is pumped through a Kelly at the top of the drill string at a pressure sufficient to flow down through the interior of the drill string to the bit at the bottom of the string and then upwardly through the annulus between the string and wall of the bore hole to remove cuttings.
From time to time, the pump is stopped and the kelly is disconnected from the drill string, for example, to add or remove pipe sections from the string, or to replace the bit, which requires pulling the entire string. A considerable amount of drilling mud remains in the kelly and can flow or drain from the lower end of the kelly when it is disconnected from the drill string.
Mud draining from the Kelly usually spills on the floor of the drilling rig and causes unsafe conditions for workmen, and can cause pollution if the mud flows along the ground. Time is lost because it is usually necessary for the workmen to wait until the mud has drained from the Kelly before another connection can be made to the drill string, and the mud lost is expensive.
During drilling of a 20,000 foot well, the drill string is disconnected from the Kelly about 800 times. Depending on the diameter of the string several thousand gallons of mud could be lost.
Mechanical Kelly valves with metal valve parts exposed to the abrasive mud are not satisfactory because they wear rapidly, often lock up when clogged with mud, and usually have parts or projections which prevent tools from being pumped through the valve into the well.
Kelly valves with a flexible tubular body, which closes automatically under the action of compressed fluid in a chamber surrounding the body are known, as described for example, in U. S. Pat. No. 4,303,100.
While the valve of U.S. Pat. No. 4,303,100 may operate satisfactorily when drilling relatively shallow wells of a few thousand feet, the flexible body is damaged at the very high pump pressures required for deep drilling, and the valve is then useless. In particular, these very high pressures in the drill string extrude the diaphragm material through the small openings in the back up sleeve and the diaphragm either punctures or fastens itself to the backup sleeve so it will not close when pumping pressure is released.
As the drilling depth increases, more pump pressure is required to pump the mud through the well, and pressures on the order of several thousand psi are not unusual for deep wells.